Heat treatment method of aluminum alloy panel

ABSTRACT

The present invention provides a heat treatment method of an aluminum alloy panel, which can prevent surface curvature of molded aluminum alloy panel. For this purpose, the present heat treatment methods comprise: cold rolling an aluminum alloy panel at a reduction ratio of 45 to 50% at a final pass in a cold rolling process; first heat-treating the cold-rolled aluminum alloy panel at 450 to 510° C. for 3 hours; rapidly cooling the heat-treated aluminum alloy panel at a rate 60° C./sec or higher after the first heat treatment; and second heat-treating the rapidly cooled aluminum alloy panel at 200 to 220° C.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims under 35 U.S.C. §119(a) the benefit of KoreanPatent Application No. 10-2008-0070424 filed Jul. 21, 2008, the entirecontents of which are incorporated herein by reference.

BACKGROUND

(a) Technical Field

The present invention relates to a heat treatment method of an aluminumalloy panel. More particularly, the present invention relates to a heattreatment method of an aluminum alloy panel, which can prevent theformation of surface curvature that can occur when the aluminum alloypanel is molded.

(b) Background Art

Aluminum alloys used for vehicle outer panels are 5xxx(aluminum-magnesium) and 6xxx (aluminum-magnesium-silicon) seriesalloys, and the panels are formed to have a thickness up to 2 mm toincrease strength and hardness.

In terms of moldability, the aluminum-magnesium alloys are superior tothe aluminum-magnesium-silicon alloys and are applied to inner and outerpanels having a complicated shape.

However, the aluminum-magnesium alloy sheets have a problem in that theyshow stretcher-strain mark formed on the surfaces thereof due to theinteraction between dislocations that cause plastic deformation andprecipitates of Al—Si during stamping or other deformations, i.e. due todynamic strain aging and inhomogeneous deformation around precipitatesby the precipitation of the alloy elements added to obtain highelongation and high formability by straining during deformation, thusdeteriorating the surface quality of the sheets.

As shown in FIG. 1, the surface waviness can be characterized by theoccurrence of serrated flow on a tension curve from tensile test onpanel materials.

A prior art method to solve the problem of surface waviness is tosubject an outer panel having such surface waviness to a post-process ofsanding the front surface of the outer panel. The method, however,reduces the productivity and increases the manufacturing cost.

In order to achieve high moldability, it is necessary to increase theadded amount of Mg; however, higher Mg content increases surfacewaviness. Accordingly, in a prior art method, in the case of the innerpanel having a complicated shape, an alloy containing 2.8% or more of Mgis used to ensure the moldability and, in the case of the outer panel,an alloy containing Mg of less than 2.8% is used to maintain the surfacequality. However, the moldability of the outer panel is not enough toaccommodate design needs. Accordingly, the outer panel is prepared byusing an aluminum-magnesium alloy containing a high content of Mgcoupled with a pose-process of sanding the front surface of the panel.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY OF THE DISCLOSURE

In one aspect, the present invention provides a heat treatment method ofan aluminum alloy panel, the method comprising: cold rolling an aluminumalloy panel at a reduction ratio of 45 to 50% at a final pass in a coldrolling process; first heat-treating the cold-rolled aluminum alloypanel at 450 to 510° C. for 3 hours; rapidly cooling the heat-treatedaluminum alloy panel at a rate more than 60° C./sec after the first heattreatment; and second heat-treating the rapidly cooled aluminum alloypanel at 200 to 220° C.

In a preferred embodiment, the aluminum alloy panel is an AA5454aluminum-magnesium alloy panel comprising 95.35-96.45 wt % of aluminum(Al), 3.0 to 3.8 wt % of magnesium (Mg), 0.2 to 0.5 wt % of manganese(Mn), 0.35 wt % of iron (Fe).

In another preferred embodiment, the aluminum alloy panel is cold-rolledat a reduction ratio of 45 to 50% such that shear stress is applied tothe surface of the aluminum alloy panel and shear texture {001}<110> isdeveloped.

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g. fuels derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example bothgasoline-powered and electric-powered vehicles.

The above and other features of the invention are discussed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will now bedescribed in detail with reference to certain exemplary embodimentsthereof illustrated the accompanying drawings which are givenhereinbelow by way of illustration only, and thus are not limitative ofthe present invention, and wherein:

FIG. 1 is a graph showing the formation of surface waviness, obtained asa result of a tensile test for an aluminum alloy panel prepared by aconventional heat treatment method;

FIG. 2 is a diagram showing textures shown as (111) pole figures aftercold rolling of a panel during heat treatment of the present invention;

FIG. 3 is a graph showing the results of tensile deformation behaviortest for an aluminum panel prepared by a heat treatment method of thepresent invention and an aluminum panel prepared by a conventional heattreatment method; and

FIG. 4 shows images that compare surface waviness of the aluminum panelprepared by the heat treatment method of the present invention and thatof the aluminum panel prepared by the conventional heat treatmentmethod.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variouspreferred features illustrative of the basic principles of theinvention. The specific design features of the present invention asdisclosed herein, including, for example, specific dimensions,orientations, locations, and shapes will be determined in part by theparticular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent partsof the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Hereinafter reference will now be made in detail to various embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings and described below. While the invention will bedescribed in conjunction with exemplary embodiments, it will beunderstood that present description is not intended to limit theinvention to those exemplary embodiments. On the contrary, the inventionis intended to cover not only the exemplary embodiments, but alsovarious alternatives, modifications, equivalents and other embodiments,which may be included within the spirit and scope of the invention asdefined by the appended claims.

The present invention provides a heat treatment method in which a simpleheat treatment process is applied to an aluminum-magnesium alloy panelin order to reduce surface waviness formed by dynamic strain aging,facilitate the molding process, and obtain high strength characteristicsby work hardening after molding.

In other words, the present invention provides a heat treatment method,which can obtain high strength characteristics after heat treatment dueto the formation of precipitates by magnesium (Mg) in analuminum-magnesium alloy panel and reduce surface roughness during themolding of the panel by performing a heat treatment in a specifictemperature range.

In an embodiment, AA5454 aluminum-magnesium alloy may be used. TheAA5454 aluminum-magnesium alloy is a commercially available alloycomprising 95.35-96.45 wt % of aluminum (Al), 3.0 to 3.8 wt % ofmagnesium (Mg), 0.2 to 0.5 wt % of manganese (Mn), 0.35 wt % of iron(Fe).

An AA5454 aluminum-magnesium alloy panel having the above-describedcomposition may be manufactured by a known method. The method comprises:dissolving a raw material in an ingot state and forming analuminum-magnesium alloy slab having a thickness of 150 mm by DCcasting; performing hot rolling at 550° C. to be rolled into a thicknessof 20 mm; cold rolling the hot-rolled coil at 420° C. to be rolled intoa thickness of 1.0 mm; and performing aging treatment at 450-510° C. for5 to 7 hours.

Here, the aging treatment contributes to increase the moldability of thepanel and maximize the elongation; however, it is inevitable that themagnesium precipitates, which cause the formation of surface waviness,become coarse.

According to the present invention, a maximum shear stress is appliedduring the cold rolling in order to suppress the growth of magnesiumprecipitates, thereby preventing the magnesium precipitates frombecoming coarse, and a multistage heat treatment is performed so thatthe precipitates are minutely distributed.

The cold rolling process is performed in five passes. The surface shearstrain is applied at the final pass of the cold rolling process, what wecalled shear rolling, applying a rolling thickness strain correspondingto 45 to 50% with respect to the thickness of the previous pass (afterfourth pass).

If the shear stress is applied in all passes, grain orientation ischanged to {001}<110>, called shear texture, and thereby the anisotropyis increased. Accordingly, it is necessary that the shear stress shouldbe applied in the final rolling pass.

In an embodiment, the AA5454 aluminum alloy panel is cold-rolled at arolling reduction ratio of 45 to 50% at the final pass, the cold-rolledpanel is subjected to a first heat treatment process which is performedat 450 to 510° C. for 3 hours, the panel then is rapidly cooled at arate of 60° C./sec or higher, and the rapidly cooled panel is subjectedto a second heat treatment process which is performed at 200 to 220° C.

That is, the AA5454 aluminum alloy panel is cold-rolled at a thresholdtemperature, where magnesium (Mg) precipitated at the grain boundary ofthe AA5454 aluminum alloy does not grow, any already-formed magnesiumprecipitates are dissolved by the first heat treatment process, and themagnesium precipitates are finely reprecipitated by the second heattreatment while being prevented from growing.

FIG. 2 is a diagram showing textures shown as (111) pole figures aftercold rolling of the panel. It can be seen from FIG. 2 that while a panelprepared by a conventional heat treatment method (A) has ring-shapedtexture on the surface and in the inside thereof, a panel prepared bythe present heat treatment method (B) has a shear texture {001}<110>developed on the surface thereof, which delays the growth of magnesiumprecipitates during the heat treatment and a ring-shaped texture, atypical rolling texture, developed in the inside thereof.

FIG. 3 is a graph showing the results of tensile deformation behaviortest for an aluminum panel prepared by a heat treatment method of thepresent invention and an aluminum panel prepared by a conventional heattreatment method. It can be seen from FIG. 3 that unlike the panelsprepared by conventional methods (heat-treated at 450° C. or 510° C. for5 hours), the panel prepared by the present heat to treatment methoddoes not show strain aging, which may mean that the growth of magnesiumprecipitates is suppressed by the second heat treatment.

Here, the effect according to the heat treatment of the presentinvention will be described with respect to Test Examples below.

Test Examples

Strain aging was examined for the panels prepared by the heat treatmentmethod according to the present invention and those prepared byconventional heat treatment methods. The formation of strain aging waschecked on tensile curves and examined through surface roughnessanalysis. The test results are shown in the following Table 1.

TABLE 1 Reduction First Second Surface Cooling rate ratio (%) inannealing First annealing Second waviness (° C./sec) final coldtemperature annealing temperature annealing after tensile after firstrolling (° C.) time (hr) (° C.) time (hr) test annealing Example 1 45450 3 200 2 Not occurred 60 Example 2 45 480 3 200 2 Not occurred 60Example 3 45 510 3 200 2 Not occurred 60 Example 4 45 450 3 200 3 Notoccurred 60 Example 5 45 480 3 200 3 Not occurred 60 Example 6 45 510 3200 3 Not occurred 60 Example 7 45 450 3 220 2 Not occurred 60 Example 845 480 3 220 2 Not occurred 60 Example 9 45 510 3 220 2 Not occurred 60Example 10 45 450 3 220 3 Not occurred 60 Example 11 45 480 3 220 3 Notoccurred 60 Example 12 45 510 3 220 3 Not occurred 60 Example 13 50 4503 200 2 Not occurred 60 Example 14 50 480 3 200 2 Not occurred 60Example 15 50 510 3 200 2 Not occurred 60 Example 16 50 450 3 200 3 Notoccurred 60 Example 17 50 480 3 200 3 Not occurred 60 Example 18 50 5103 200 3 Not occurred 60 Example 19 50 450 3 220 2 Not occurred 60Example 20 50 480 3 220 2 Not occurred 60 Example 21 50 510 3 220 2 Notoccurred 60 Example 22 50 450 3 220 3 Not occurred 60 Example 23 50 4803 220 3 Not occurred 60 Example 24 50 510 3 220 3 Not occurred 60Comparative 30 450 5 — — Occurred Example 1 Comparative 30 480 5 — —Occurred Example 2 Comparative 30 510 5 — — Occurred Example 3Comparative 30 450 7 — — Occurred Example 4 Comparative 30 480 7 — —Occurred Example 5 Comparative 30 510 7 — — Occurred Example 6Comparative 30 450 3 200 2 Occurred 60 Example 7 Comparative 30 510 3200 2 Occurred 60 Example 8 Comparative 30 450 3 200 3 Occurred 60Example 9 Comparative 30 510 3 200 3 Occurred 60 Example 10 Comparative30 450 3 220 2 Occurred 60 Example 11 Comparative 30 510 3 220 2Occurred 60 Example 12 Comparative 30 450 3 220 3 Occurred 60 Example 13Comparative 30 510 3 220 3 Occurred 60 Example 14 Comparative 45 430 3200 2 Occurred 60 Example 15 Comparative 45 430 3 200 3 Occurred 60Example 16 Comparative 45 530 3 200 2 Occurred 60 Example 17 Comparative45 530 3 200 3 Occurred 60 Example 18 Comparative 50 430 3 220 2Occurred 60 Example 19 Comparative 50 430 3 220 3 Occurred 60 Example 20Comparative 50 530 3 220 2 Occurred 60 Example 21 Comparative 50 530 3220 3 Occurred 60 Example 22 Comparative 55 450 3 200 2 Surface 60Example 23 defect Comparative 55 450 3 200 3 Surface 60 Example 24defect Comparative 55 510 3 200 2 Surface 60 Example 25 defectComparative 55 510 3 200 3 Surface 60 Example 26 defect Comparative 55450 3 220 2 Surface 60 Example 27 defect Comparative 55 450 3 220 3Surface 60 Example 28 defect Comparative 55 510 3 220 2 Surface 60Example 29 defect Comparative 55 510 3 220 3 Surface 60 Example 30defect Comparative 45 450 3 200 2 Occurred 50 Example 31 Comparative 45450 3 200 3 Occurred 50 Example 32 Comparative 45 510 3 200 2 Occurred50 Example 33 Comparative 45 510 3 200 3 Occurred 50 Example 34Comparative 50 450 3 220 2 Occurred 50 Example 35 Comparative 50 450 3220 3 Occurred 50 Example 36 Comparative 50 510 3 220 2 Occurred 50Example 37 Comparative 50 510 3 220 3 Occurred 50 Example 38

It can be seen from Table 1 that no surface waviness occurred inExamples 1 to 24 in accordance with the present invention (B) whereassurface defects or surface waviness occurred in Comparative Examples 1to 38 (A).

As described above, according to the present methods, magnesiumprecipitates can be prevented from becoming coarse, surface wavinessformed by dynamic strain aging during molding of the panel can therebybe reduced, which makes it possible to facilitate the molding processand obtain high strength characteristics by work hardening aftermolding.

The invention has been described in detail with reference to preferredembodiments thereof. However, it will be appreciated by those skilled inthe art that changes may be made in these embodiments without departingfrom the principles and spirit of the invention, the scope of which isdefined in the appended claims and their equivalents.

1. A heat treatment method of an aluminum alloy panel, the methodcomprising: cold rolling an aluminum alloy panel at a reduction ratio of45 to 50% at a final pass in a cold rolling process; first heat-treatingthe cold-rolled aluminum alloy panel at 450 to 510° C. for 3 hours;rapidly cooling the heat-treated aluminum alloy panel at a rate of 60°C./sec or higher after the first heat treatment; and secondheat-treating the rapidly cooled aluminum alloy panel at 200 to 220° C.2. The method of claim 1, wherein the aluminum alloy panel is an AA5454aluminum-magnesium alloy panel comprising 95.35-96.45 wt % aluminum(Al), 3.0 to 3.8 wt % of magnesium (Mg), 0.2 to 0.5 wt % of manganese(Mn), and 0.35 wt % of iron (Fe).
 3. The method of claim 1, wherein thealuminum alloy panel is cold-rolled at a reduction ratio of 45 to 50%such that shear stress is applied to the surface of the aluminum alloypanel and shear texture {001}110> is developed.